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Posted By Topic: Main Earth

Sparkyman
May 17 2013 07:06

Just a quick clarification on a simple but often miss-understood scenario that over the years Ive had various answers from various inspectors etc.
In Christchurch we have to run a continuous 6mm earth from S/B unbroken to Earth stake, normal you say.
Sometimes it runs through the Metal meter box but we are not allowed to loop it on the Metal Meterbox on the way through (or at least brake it), normal again you say.
But to save cable if the S/B is miles away often the Meterbox earth goes to the closeer Earth Stake instead of the SW/B.
So the Earth stake has 2 clamps on it etc.
Question is some inspectors say OK and some reject it as they say all Main and Bonds need to go back to S/B so they are connecting with main MEN etc.
Comments is earthing Meterbox at main Earth legal??
   

CraigG
May 17 2013 07:24

Bonding the meter box from the stake is OK if you use a sep clamp.

A main earth must run directly from the earthbar to the stake , not to the MB First.
   

pluto
May 17 2013 08:50

Could someone tell me in the Wiring Rules that the above practice is detailed?

It is NOT in my copy of the Wiring Rules!
   

pluto
May 17 2013 08:51

Could someone tell me in the Wiring Rules that the above practice is detailed?

It is NOT in my copy of the Wiring Rules!
   

AlecK
May 17 2013 10:11

Nor mine!

1
There is nothing to prohibit an MEC from going through a meterbox, as long as that is on the most direct route from MSB to electrode. As it might be if the electrode is below meterbox.

2
nothing requires the MEC to be unbroken. though it is good practice to avoid joints that can be undone. They oly need to e continuous in the sense of 0.5 ohms max.

3
the green conductor from the meterbox is NOT bonding, it's earthing; designed to carry fault current.

4
regardless of whether a particular conductor is earthing or bonding, it is NOT ok to connect it to the electrode. the only "exception" is for bonds to waterpipes only, which may be an extension of the MEC - but NOT a separate clamp
(prior to 2003 it was permitted to connect bonds to the electrode)

5
anyone who thinks all PECs and bonds must go back to earth bar has not read the book since 2002. Good practice; yes. It makes testing easier. Mandatory; no.

   

Rhino
Apr 29 2017 14:09

Aleck - regards to connecting meter box earth bond directly to to earth stake
4. Saying it's not OK to do so
Then
5. It's good practice to connect to MSB earth bar however not mandatory?
So is it ok to connect meter box bond directly to earth stake instead of going to main earth bar ? Will this pass inspection ?
   

megabyting
Apr 29 2017 14:24

Mandated the main earth shall be fastened to the main earth bar in the MEN switchboard with double lock nuts and the link shall be the most direct path to the earth electrode with properly labelled "Main earth do not remove.. something like that.""

   

pluto
Apr 29 2017 14:43

Look at the fig in Section 5 of AS/NZS 3000 for the permitted connections to the main earthing conductor to a main switchboard, distribution switchboards.

The example being quoted sounds like an 'inspectors personal good idea"and not supported by ESR 2010 or AS/NZS 3000.
   

Sarmajor
Apr 29 2017 16:15

A metal meter box must be earthed. The earthing wire can go to the nearest available earth bar. It does not have to go to the main switchboard.
AS/NZS3000:2007 section 5 fig 5.3 gives acceptable examples for connections to earth for bonding and earthing. Note there is no second connection to the earth electrode shown in the figure.

The main earth conductor must run from the earth electrode via the most direct path to the main switchboard earth bar. This could be through the meter box if it was a suitable path.

As Aleck has stated the main earth conductor must be electrically continuous. This does not mean that you couldn't make a crimp connection that just happened to be adjacent to the bonding stud in the meter box and earth the meter box.

As per fig 5.3 you can connect equipment to the main earth conductor. The important thing is that disconnecting the bond from the meter box must not break the main earth conductor?

CraigG
Your answer directly contradicts the methods shown in fig 5.3 and is not acceptable.

megabyting
Your comments about double locknuts are not based on any information that is currently published in any of the regulations or standards that we use.
The only requirement for a double screw fastening is in AS/NZS3000:2007 2.9.4.2 & 3.7.2.11
The fundamental requirement is that the connections are secure and stay secure under working conditions.



   

Rhino
Apr 29 2017 22:44

Samajor;

As per fig 5.3 you can connect equipment to the main earth conductor. The important thing is that disconnecting the bond from the meter box must not break the main earth conductor?

Bonding meter box;
Based on that running the main earthing conductor through the meter box connecting to the meter box by means of a lug and then continuing the cable to the earth electrode is acceptable?

And leaving the main earthing conductor as is ie" direct to electrode" and bonding the meter box by means of a separate earth bonding conductor that runs from meter box directly to earth electrode is unacceptable?
   

DougP
Apr 30 2017 06:56

For my installs, the most direct route for the MEC is through the meter box. The 6mm2 earth wire for the meter box is then attached to the MEC with a 6mm line tap.

The MEC can't be directly connected to the meter box earth terminal, as per 5.5.1.1
   

Sarmajor
Apr 30 2017 19:00

Rhino,
Have a look at fig 5.3 and you will see that a second earth connection to the main earth electrode is not shown as a solution.

I have seen hundreds of meter boxes earthed using a crimp lug to maintain the continuity of the main earth conductor as I described earlier.

The method suggested by Dougp would appear to be a better method and removes the argument about continuous versus continuity for the main earth conductor and I like it.

With regard to this statement
'And leaving the main earthing conductor as is ie" direct to electrode" and bonding the meter box by means of a separate earth bonding conductor that runs from meter box directly to earth electrode is unacceptable'

I don't make the rules and this solution is not shown as a acceptable method in fig 5.3.

Surely you have come across installations with the main earthing conductor disconnected from the earth electrode.
Using the second earth clamp on the electrode to bond the metal meter box increases the risk that the earth connection will be compromised by the lawn mower or the goat chewing the wire off.

This leaves the metal meter box with no earth connection. It also leaves the installation without an earth connection but the installation still has a path for fault current via the PEN (Neutral)In the mains.
The meter box has nothing once the connection to the earth electrode is lost.
That is why a connection to an earth bar in the installation is one of the solutions shown in fig 5.3.

Connection to the main earth conductor is another option and Dougp's method sounds like a suitable method

I actually suspect that this problem usually comes from the electrician forgetting to run a earth wire from the switchboard back to the meter box during pre wire.
   

Apprentice
Apr 30 2017 21:02

Are you all aware of the age of this thread? It seems rather odd to bring up something from 4 YEARS AGO and challenge it as though it was posted yesterday.
   

Apprentice
Apr 30 2017 21:05

Looking at you Rhino!
   

zl2aj
May 01 2017 09:36

The earthing system at the meter box is still TNC at this point. It doesnt become TNC-S until the switchboard.

This may sound like heresy but with the above in mind, the main neutral conductor is also the earthing conductor at this point (as it is in works).

The above was suggested as a solution by a senior person in worksafe.
   

Sarmajor
May 01 2017 19:58

And will be a detailed solution for earthing the metr box in the next version of AS/NZS3000.

I was in the room when the use of the PEN was put forward as a more technically correct method to earth the meter box, as were many contributors to this forum.
   

Rhino
May 02 2017 15:56

Yes thats what i referring to earthing the meter box so equipotential bonding , which i through 4mm2 was fine to use , 6mm2 used for telecommunications, but i see many people in the forum suggesting 6mm2 ? is 4mm2 acceptable just to bond meter box enclosure?
   

AlecK
May 02 2017 18:02

The green wire for a metallic meterbox has nothing to do with equipotential bonding, but is all about fault protec tion.
accordingly the process is earthing, and the conductor is a PEC and MOT a "bonding" wire.
As a PEC, the minimum size is set by Table 5.1
   

BCee1
Sep 05 2017 16:38

This is good timing. Have just come across this one today where the meter box is on the fence line then power goes to a shed. The meter box earth and peg earth wires both go to the neutral crimp link. No earth wire has been run to the house. At the house is an earth peg but its earth wire goes into a very roughly crimped 16 mm lug. We intend running an earth wire back to the house, and removing the other earths but would any other solution be acceptable at this point? We've been asked check over the house as its just been purchased.
   

BCee1
Sep 05 2017 17:34

Whoops. Forgot to add that I first thought of turning the meter box into the main board, and MEN at that point but the box isn't big enough and upgrading to a bigger box was balked at...
   

DougP
Sep 05 2017 21:41

It's difficult to work out from your description - particularly the house/shed sequence.

Plus, without knowing the age of the installation, it's hard to say what might have been compliant when it was installed.

If you want to upgrade it to the current standard, the first thing to figure out would be where the MSB is going to be. House, shed, or meter box.

As for the costs (or was it just size?) being balked at, I can't see that there is going to be a cheap solution at current standards.

Then again, there might be no reason why it can't just stay as it is?
   

BCee1
Sep 05 2017 23:39

Meter box is on the fence, then mains goes u/g under a path to a shed, then the house is subbed from the shed. Age is maybe 40 to 50 odd years? As previously mentioned could this have originally come under works as the original meters and cable would have been owned by what was then a Power Board?
   

mrsparky
Sep 06 2017 07:29

Unless I am reading that wrong this sounds fine to me rough and could do with a tidy but ok.

How I am seeing it is that the cable to the house is a sub-main with its own earth.

The main thing it sounds like this needs is making sure the earth at the house is adequately sized and run and properly terminated at the peg at the house, and that a MEN link is fitted at the board.

The other thing you could do to tidy the meter board is if possible fit some small earth-neutral bars even 4 way would be enough to terminate the 2 earths and the 2 neutrals and fit a MEN link.

Had exactly without knowing more detail but this work should be an improvement without to much additional cost.
   

AlecK
Sep 06 2017 09:35

"The meter box earth and peg earth wires both go to the neutral crimp link. "

If this is referring to the crimp link that terminates the incoming mains N at the shed switchboard, I don't see a problem. Used to be standard to do this; typically the meter shunt(s) would be in same connection. So ESR 113 probably applies

Nothing wrong even today with shed as MSB and house on PEN submain.
   

BCee1
Sep 06 2017 15:42

Appreciate the replies. Hope this doesn't muddy the waters even more. Supply from pedestal to metal meter box. Neutral's, meter shunt, earth peg, meter earth all crimped together. From there, neutral screen underground to shed. In shed MEN board with its own earth peg, then sub off to house with its Men and own earth peg. The house I've just found out was shifted to site some time ago, hence fed from shed as a sub. While not kosher today, it sounds like from the replies I can leave it as is where is.
   

AlecK
Sep 07 2017 09:15

So, in effect the meterbox is et up with MEN, but not as main switchboard; then PEN to MEN swbd in shed, then another PEN submain to MEN swbd in house.

If the meterbox was set up as main switchboard, then under 199x Regs (currently can't check exactly when)this would have been kosher. And under today's rules could be adapted to be compliant.

Not sure whether it was ever kosher to have meterbox MEN without setting it up as MSB.

current Wiring Rules would require a main switch in the meterbox; preferably but not necessarily upstream of meters. Also require separate N & E "bars", with removable link and separate terminals for incoming & each outgoing N. Plus overcurrent protective device for "submain" to shed. Plus of course comply with all other requirements for switchboards


At shed, the incoming N and the outgoing house submain N would need to be terminated together independently of their connection to shed N-bar.
   

BCee1
Sep 07 2017 14:57

Thanks again for setting us in the right direction. Will tidy up whats in the box and run an earth wire back to the MEN in the shed.